1. Field of the Invention
This invention relates to an inductive joystick and to a signal processing circuit for such an inductive joystick.
2. State of the Art
Inductive joysticks are known for controlling apparatus in many different applications. One such joystick is shown in FIG. 1 of the drawings and comprises an operating lever 16 having a primary induction coil 10 fixed to its lower end. A pair of fixed secondary coils 12,14 are provided for each axis of the joystick, the coils of each such pair being connected so that their outputs are in anti-phase. The primary coil 10 is provided with a sinusoidally varying supply voltage via flexing wires 11 which connect the supply (not shown) situated in the fixed portion of the joystick to the primary coil 10.
In use, the operating lever 16 is pivoted about a pivot point 9 such that the primary coil 10 is displaced relative to the secondary coils 12,14. When the operating lever 16 is in the central position, the magnetic coupling between the primary coil 10 and the secondary coils 12,14 causes equal output voltages to be induced in the two secondary coils. Because these output voltages are connected in anti-phase, they are cancelled out and there is no resultant output from each pair of secondary coils. However, if the operating lever 16 is moved, whereby the primary coil 10 is displaced relative to the secondary coils 12,14, the magnetic coupling between them is altered, and the output voltages of the two secondary coils are no longer equal. There will therefore be a resultant output from the pair of secondary coils on the axis along which the operating lever is moved. This resultant output, which is dependent upon the distance by which the primary coil 10 has been displaced from its central position relative to the secondary coils 12,14, is applied to a control system which can therefore determine the relative position of the operating lever, for appropriate control of the apparatus for which the joystick is provided.
A problem associated with the above-described arrangement is that the flexing wires 11 are subjected to fatigue action in operation. Thus, over a period of time, the wires may weaken and break, causing failure of the joystick.
Another known joystick, which overcomes the above problem, is shown in FIG. 2 and again comprises an operating lever 16, pivoted at 9, and a pair of fixed secondary coils 12, 14 for each axis of the joystick, the coils of each such pair being connected so that their outputs are in anti-phase. However, in this case, the primary induction coil 10 is fixed in the centre of the secondary coils, and all of the coils are wound on a ferromagnetic core 13. A ferromagnetic disc 15 is fixed to the lower end of the operating lever 16. When the lever 16 is in its central position above the coil arrangement, the coupling between the primary coil 10 and the respective secondary coils 12,14 causes equal output voltages to be induced in the secondary coils and the resultant output from each axis is accordingly zero. However, displacement of the operating lever 16 and thus of the ferromagnetic disc 15 causes a change in the magnetic coupling between the primary coil 10 and the secondary coils 12,14 of the axis in which the lever is moved. This in turn causes the output voltages induced in those secondary coils to be unequal and there is therefore a resultant output from the secondary coils in that axis. This resultant output is again applied to a control system which thereby determines the position of the lever 16 and appropriately controls the apparatus for which the joystick is provided.
In the arrangement shown in FIG. 2, the need for flexing wires is avoided because the primary coil is situated on the fixed portion of the joystick and may be connected directly to the supply voltage. However, the resultant output voltage versus operating lever deflection characteristic is not linear. Therefore, it is difficult to obtain accurate control of the apparatus. Another disadvantage of the above-described arrangement is that ferromagnetic material is expensive and the need for a large ferromagnetic core common to all of the coils results in a relatively costly arrangement.
We have now devised a joystick which overcomes the problems, outlined above, which are associated with known joysticks.